Process of recovering casing-head gasoline



Nov. 28, 1939.

IA. J. L. HUTCHINSON PROCESS 0F RECOVERING CASING-HEAD GASOLINE Filed Nov. 2, 1936 Patented Nov. 28, 1939 UNITED STATES PROCESS 0F RECOVERING CASlNG-HEAD GASOLINE Arthur John Lindsay Hutchinson, Palo Alto,

Calif., assigner to The Fluor Corporation, Lim-- ited, Los Angeles, Calif., a corporation o! Cali- Application November 2, 1936, Serial No. 108,721

8 Claims.

This invention relates to improvements in recovering gasoline from natural gas and has for its principal object the treatment of natural gas in such fashion that the same is substantially completely denuded of desired constituents and a nished liquid gasoline product is obtained of such specifications as to be readily usable in industry.

Another object is to provide a process in which initial construction costs for suitable apparatus can be immensely reduced over known processes achieving a. similar result due to the fact that such apparatus can be greatly reduced in size owing to the eiciency of the process.

A further object is to provide a process of such efciency that the operating costs of suitable apparatus are greatly reduced over those of operating -known apparatus having the same capacity.

A further object is to provide a process in which heat losses are so minimized that, in continuous operation, by the use of efficient heat exchange fuel economy is a maximum.

Further objects and results will become apparent as the process hereinafter becomes more fully disclosed.

Natural gas as it comes from wells is a mixl ture of gaseous and vaporized hydrocarbons, certain' of the latter being desired constituents of so-called casing-head gasoline and are recovered in liquid form.

The series of hydrocarbons in natural gas consists generally of the methane series of hydrocarbons which, for the purpose of illustrationherein, may be indexed as follows:

Methane C1 Ethane Cz Propane Ca Iso-butane Cn Norma1 butane Ct Isopentane C51 Normal pentane Cs Other hydrocarbons heavier than pentane occur but, for the. purpose of this invention, the

treatment of a mixture of the abovehydrocarrecover therefrom a finished casing-head gasoline with maximum economy.

More speciilcally, the process of this invention provides a casing-,head gasoline having a relatively high percentage of the heavier hydrocarbons of the mixture, such as normal butane and heavier, when compared with the original natural gas in which the hydrocarbons lighter than normal butane predominate.

Further, the process of this invention provides a casing-head gasoline which contains no methane, ethane, or propane. and a relatively small percentage of iso-butane.

Referring to the diagrammatic sheet of drawings which illustrates an operating process, natural gas is introduced through pipe I0 into an absorption tower, or absorber, Il of any well known type wherein selective absorptionof de- 'sired constituents of the gas takes place.

Considering the process as operating continuously, the natural gas iiow was at the rate of 65,000,000 cubic feet per twenty-four hours and had the following composition and percentages:

Mol.

process is exible and pressures and temperatures may be varied with either increase or decrease to any desired amount depending on atmospheric and operating requirement at the plant location. 'Gas pipe lines are operated at pressures varying from about 100# to 800# per square inch and where cooling from 90 F. to 45 F. may be obtained. This present process can be advantageously used under all these conditions. It is understood that the oil rates must be correctly chosen for the operating pressures and temperatures in accordance with well known methods.`

From the top of tower IlI undesired constituents generally termed Wild gases pass out through pipe I 2 controlled by back pressure regulator I3 at a temperature of about 75 F. and comprise 64,500,000 cubic feet of gas as follows:

Mol. C1 86.25

C3 3.73 Cn"- 0.23

The absorption oil containing the bulk of the desired constituents together with some undesired constituents passes out of the bottom of tower II, controlled by liquid level regulator I4, at temperature of about 90 F. through pipe I5 to a vent tank I6 and comprises the following mixture and percentages:

Mol. C1 12.22 C2 6.28 C3 7.40

C4 2.44 05+ 5.54 Absorption oil 64.80

Liquid level regulator I4 acts as a pressure regulator on pipe I5 to reduce the pressure in Vent tank I 6 to about 225 pounds per square inch whereupon certain gaseous and vaporous `constituents pass off through pipe II, controlled by back lpressure regulator I8, to a stripper I9. These fractions are as follows:

Mol. C1 80.40 Cz 12.09 C3 6.15 C41 0.42 C4 0.64 C5+ 0.32

and together comprise 358,000 cubic feet.

Liquid hydrocarbons of a gravity of about 45 A. P. I. composed of the following:

Mol. C1 8.04 Cn 5.94 C3 '7.47

C41 1.38 C4 2.45 C5+ 51.86 Absorption oil 68.90

of about 36 A. P. I. is withdrawn from the bottom at a temperature of about 375 F. and at the rate of about 218 gallons per minute through pipe 25, controlled by liquid level regulator 26, and passes in heat exchanging relationship with the oil fed to stripper I9 through exchangers 23 and 22 and thence to an absorption oil accumulator 21a for storage and reuse in the system.

carbons pass from the top thereof at a temperature of about 380 F. through pipe 28 and heat exchanger29 wherein an output of 4,786,000 B. t. u. is eiiected to reduce the temperature to about 150 F. The liquefied and gaseous fractions then pass through a cooler 30 wherein 1,000,000 B. t. u. are exchanged causing a further drop in temperature to about F.

Any water is then removed in trap 3| and the mixture then comprising liquid hydrocarbons of about 53.6 A. P. I. gravity is discharged into the lower part of absorption tower 32.

The mixture is supplied to what is termed a low pressure absorption tower 32 at the rate of about 36 gallons per minute of the liquid together with gas at the rate of about 1,640,000 cubic feet per day.

Absorption tower 32 is operated under about 52 pounds gage pressure and is supplied with absorption oil from accumulator 21a through pipe 33, pump 34, iiow regulator 35 and cooler 36 in which latter the temperature is reduced to about 75 F.

The absorption oil containing desired constituents and of a gravity of about 45.8 A. P. I. passes from the bottom of tower 32 through pipe 21, controlled by liquid level regulator 38, at the rate of about 72 gallons per minute while at a temperature of about F. and has the following constituents:

Mol. C1 0.69 Cz 2.32

C3 9.10 C41 3.22 C4 7.52 05+ 21.05 Absorption oil 56.10

The stream is led through heat exchanger 29 by pump 39a wherein its temperature is raised to about 340 F. and then through heater 38a to maintain such temperature.

It is clear that under certain conditions of length of pipe, insulation, etc. heater 38a may be dispensed with and the oil heated solely in heat exchanger 29.

The oil, thus heated to about 340 F., is

vpassed into fractionator 39 which operates under about 55 pounds gage pressure to cleanly separate the absorption oil from the remaining constituents and said absorption oil of about 36 gravity A. P. I. is withdrawn from the bottom of fractionator 39 through pipe 40, controlled by liquid level regulator 4I, .and ows at the rate of about 55 gallons per minute while at a temperature of about 330 F. back to pipe 25 leading from stripper I9 and the thus commingled streams ow at the rate of about 273 gallons per minute while at a temperature of about 366 F. through heat exchangers 23 and 22 to accumulator 21a for reuse.

Fractionator 39 is supplied with about 650 pounds of steam per hour through pipe 42a and is equipped with a bottom heater 42, which may be the usual closed steam coil, and the gaseous and vaporized constituents at a temperature of about 200 F. pass from the upper part thereof through pipe 43, and cooler 44 to areux condensate tank 45 at a temperature of about 75 F. Any gaseous 'and vaporized hydrocarbons are taken from the top of reflux tank 45 and passed through pipe 45a back to pipe 28 prior to the latters discharge into absorber 32 so that there may be as complete a recovery as ypossible of desired constituents. Tower 32 thus functions as both a primary low pressure absorber anda reabsorber.

Some of said reflux condensate at a tempera` 110.8 A. P.`I. and comprises the following:

. Mol. C1 0.23 Cz 2.97 2 c3 18.63 C41- 7.41 C4. 17.96 C5+ 52.70 25 The thus heated reflux condensate is treated in stabilizer 5I at about 200 pounds gage pressure, said stabilizer being equipped with a bottom heater 52, wherein undesired constituents are finally separated to pass out through pipe 53 30 at a temperature of about 135 F. to be cooled to about 75 F. in cooler 54 and received in reflux condensate tank 55.

A portion of this last-named reflux condensate is withdrawn through pipe 56 governed by flow regulator 51, and used as a reflux medium in stabilizer 5I at the rate of about 17.5 gallons per minute, while a second portion may be withdrawn, if desired, through level regulator 59.

The desired finished gasoline is passed at a 40 temperature of about 270 F. through pipe 59,

controlled by liquid level regulator 60, heat ex` changer 50, and cooler 6I at a temperature of about 75 F. to a storage tank 62 at the rate of about 19,274 gallons per day, gravity about 100 45 A. P. I. consisting of C41 7.60 mol.'%; Gr.40 vol. %4 C4 23.40 mol. 19.24 vol. C5+ 69.00 mol. 74.36 vol.

Coming now to an important feature of the 50 invention, undesired constituents consisting of:

Mol.

55 C3 g 79.55 C41- 6.74

are taken from tank 55 through pipe 63 and back pressure regulator 64 to a header 65 into Such combined stream of constituents undesired l in the finalrproduct is passed into the bottom of a so-called saturating absorber 61, supplied with absorbent oil from accumulator 21a by means of pipe 68, pump 69, ow regulator .10,1 andcoolerv 1I at the rate of about 242.gallons per minute, y 75l/its temperature being reduced by cooler 1I from which is likewise led undesired constituents quired.

about 125 F. to about 80 F. while the saturating absorber operates under a pressure of about 50 pounds gage.

In high pressure lsystems for treating natural gas, it is important to separate desired constituents and conserve all the gas. Therefore, the residue gases from pipe 65 are passed into saturating absorber 61 wherein about 50% of all such gases are absorbed, or practically everything but methane and ethane which may be Withdrawn through pipe 12 at the rate of about 1,081,000 cubic feet per day in the following proportions:

Mol. C1 71.50 Cz 28.50

and may be used for fuel.

The remaining gas absorbed in the oil passes through pipe 13, controlled by level regulator 14, pump 15, and cooler 16, at' a temperature of about 75 F. and at the rate of 254 gallons per minute, said oil being about a gravity of 39.6 A. P. I., to absorber II.

The gravity of the oil from saturating absorber 61 will vary with the composition of the gas absorbed as well as with the temperature and pressure of the absorption step and, for commercial operations, will range from about 33 A. P. I. to 45 A. P. I., but the gravity will always be materially increased over the gravity of the hydrocarbon oil originally used for absorptionas, for instance, coming from stripper I9 and fractionator 39.

As illustrative of the described process, the oil from absorber 61 has the following mol. percentages:

Mol. C1 0.7 C2 2.14 C3'- 9.79 CM 0.81 C4- 0.80 Absorption oil 85.76

' which causes an increase in gravity of. the absorption oil from 36 A. P. I.` to 39.6 A. P. I.

The cycle of operations from absorber II has been described, but the function of saturating absorber 61 in the system is highly important because as the pressure is stepped up by pump 15, this pump can handle the absorption oil with its ab orbed gases and eliminates the use of a recompressor.l y

By reason of this action the number of liquid mols entering and leaving absorber I I are nearer the same than if absorbent oil alone was used and thus better absorption of nally desired constituents is obtained. y

Again, the load on-absorber II is materially reduced by the use of saturating absorber 61 because only constituents desired in the nal product are picked up in absorber II hencev the `evaporation up to a pointl where vsome of the heavier absorption oil will be carried over.

The `system vis applicable ,to bothA high and` low pressure gasoline recovery plliirlts.` When 1ow pressures of the order of 25 to 100 pounds are used in absorber H, the stripper I9 may be bypassed and the rich absorption oil taken directly to fractionator A39 through suitable heat exchangers.

Likewise, liquid reux from tank 55 may be taken to be introduced into saturating absorber 61 together' with gaseous hydrocarbons to the same effect as described.

The plant, as described, has greater exibility in producing iinished gasoline at varying speciilcations over present known absorption plants, particularly when the recovery of pentane and heavier hydrocarbons in the gas together with varying percentages of butane is desired, practically all methane, ethane, and propane being eliminated.

The described system, in addition to exibility, carries such heat load as to greatly reduce initial investment costs as well as operating costs.

Due to its efficiency, the steam boiler capacity is greatly reduced as undesired constituents are largely eliminated before fractionation and therefore the fractionator and stabilizer can be of minimum size. This reflects also on the coolers which, therefore, can be smaller due to lowered heat load.

As an illustration by comparative figures, the heat load for the above described system is about 8,481,000 B. t. u. per hour, steam 10,200 pounds per hour. A plant of equal capacity according to the present best known design will require about 11,000,000 to 12,000,000 B. t. u. per hour.

Whenit is known that on ordinary pipe line absorption systems steam costs about 25 cents per million B. t. u. per hour using natural gas for fuel, the dilerential in this respect alone in favor of the present invention is apparent, especially in this art where high economy in costs is necessary to operate in a highly competitive industry.

It should be understood that where live steam has been used herein, it acts mainly by partial pressure elects to strip the liquids thoroughly and not particularly as a heat transfer agent. Being chemically inert as to the hydrocarbons, it is possible at times to replace the steam with other lnert gases for stripping purposes such as nitrogen, carbon dioxide and the'like, but such is not generally desirable due to the lower specic heat of such other inert gases.

As applied herein, the mol. per cent means the relative number of molecules of each constituent, or group of constituents, present in a hydrocarbon mixture whether in the liquid or Vapor phase. For instance, a 36 A. P. I. oil of 200 mol. weight designates an oil having 0.0352 mol. per gallon or one in which 13.4 standard cubic feet of vapor would condense to .'orm one gallon of liquid. This designation is particularly adapted to the absorption oil of 36 A. P. I. gravity referred to herein which has a mean molecular weight of about 200'.

I claim as my invention:

1. A process of treating natural gas which com-- bons from the system, decreasing the pressure and increasing the temperature on said absorption oil to completely remove all absorbed hydrocarbons, condensing and stabilizing said removed hydrocarbons to recover a liquid fraction containing some iso-butane and a relatively large percentage of hydrocarbons heavier than iso-butane also a gaseous fraction containing some iso-butane and a relatively large percentage of hydrocarbons lighter than iso-butane, passing said gaseous fraction through absorption oil under lower pressure conditions than said first-mentioned absorption oil to absorb substantially all propane and heavier hydrocarbons while methane and ethane are substantially unabsorbed, then stepping up the pressure on said last absorption oil to contact said original stream of gas and complete the cycle. i

2. A process of treating natural gas which comprises: iiowing a stream of 'natural gas containing hydrocarbons of the methane series, such as pentane and lighter and pentane and heavier, through absorption oil under such conditions of temperature and pressure that the greater part of the hydrocarbons lighter than butane are unabsorbed, removing such unabsorbed hydrocarbons from thesystem, decreasing the pressure and increasing the temperature on said absorption oil to completely remove all absorbed hydrocarbons by steps including fractionation, condensing the heavier portion of said fractionated hydrocarbons and stabilizing same to recover a liquid fraction containing some iso-butane and a relatively large percentage of hydrocarbons heavier than lso-butane also, a gaseous fraction containing some iso-butane and a relatively large percentage of hydrocarbons lighter than iso-butane, passing said gaseous fraction through absorption oil under lower pressure conditions than said first-mentioned absorption oil to absorb substantially all propane and heavier hydrocarbons while methane and ethane are substantially unabsorbed, then stepping up the pressure on said last absorption oil to contact said original stream of gas and complete the cycle.

3. A process of treating natural gas which comprises: flowing a stream of natural gas containing hydrocarbons of the methane series, such as pentane and lighter and pentane and heavier, through absorption oil under such conditions of temperature and pressure that the greater part oi' the hydrocarbons lighter than butane are unabsorbed, removing such unabsorbed hydrocarbons from the system, decreasing the pressure and increasing the temperature on said absorption oil to completely remove all absorbed hydrocarbons and stripping the same with an inert gas, cooling said stripped hydrocarbons and condensing part thereof, passing the vaporized part of the cooled hydrocarbons through further absorption oil to reabsorb a liquid fraction and separate out a gaseousfraction, lpassing said absorption oil containing said reabsorbed liquid fraction first in heat exchange relation with said stripped hydrocarbons to cool such hydrocarbons then into a fractionator to completely remove all reabsorbed hydrocarbons, stabilizing said removed hydrocarbons to recover a liquid fraction containing some iso-butane and a relatively large percentage of hydrocarbons h'eavier than iso-butane also a gaseous fraction, containing some isobutane and a relatively large percentage of hydrocarbons lighter than iso-butane, passing said gaseous fraction through absorption oil under lower pressure conditions than said first-mentioned absorption oil to absorb substantially all propane and heavier hydrocarbons while methane and ethane are substantially unabsorbed, then stepping up the pressure on said last absorption oil to contact said original stream of gas and complete the cycle.

i 4. A process of treating natural gas which comprises: owing a stream of natural gas containing hydrocarbons of the methane series, suchv as pentane and vlighter and pentane and heavier, through absorption oil under such conditions of temperature and pressure that the greater part of the hydrocarbons lighter than butane areunabsorbed, removing such unabsorbed hydrocarbons from the system, decreasing the pressure and increasing the temperatur'e on said absorption oil to completely remove all absorbed hydrocarbons and stripping the same with an inert gas, cooling said stripped hydrocarbons and condensing part thereof, passing .the vaporized part of the cooled hydrocarbons through further absorptionf'oil to reabsorb a liquid fraction and separate' out a gaseous fraction, passing said absorption oil containing said reabsorbed liquid fraction rst in heat exchange relation with said stripped hydrocarbons to cool such hydrocarbons then into a fractionator to completely remove all reabsorbed hydrocarbons, stabilizing said removed hydrocarbons to recover a liquid fraction contain'- ing some iso-butane and a relatively large percentage of hydrocarbons heavier than iso-butane also a gaseous fraction containing some viso-butane and a relatively large percentage of hydrocarbons lighter than iso-butane, passing said gaseous fraction together with the gaseous fraction separated from said reabsorption oil through absorption oil under lower pressure conditions than said first-mentioned absorption oil to absorb substantially all propane and heavier hydrocarbons while methane and ethane are substantially unabsorbed, then stepping up the pressure on said last absorption oil to contact said original stream of gas and complete the cycle.

5. IIn a process for recovering casing-head gasoline from natural gas containing hydrocarbons of the methane series including butane and lighter hydrocarbons in which the gas is contacted with absorption oil under such conditions of temper'- ature and pressure that propane and heavier hydrocarbons are absorbed but the major portion of the gas remains unabsorbed, the enriched absorption oil is then treated to completely remove all absorbed hydrocarbons and to recover a liquid fraction and a gaseous fraction from said removed hydrocarbons, that combination of steps-which comprises: passing said gaseous fraction as a gas through absorption oil under conditions of temperature and pressure that propane and heavier hydrocarbons are absorbedi therein while methane and ethane are but slightly absorbed, then stepping up the pressure on such oil last mentioned and contacting the same with said original gas.

6. In a process for recovering casing-head gasoline from natural gas containing hydrocarbons of the methane series including butane and lighter hydrocarbons in which the gas is contacted with absorption oil under such conditions of temperature and pressure that propane and heavier hydrocarbons are absorbed butthe maior portion f the gas remains unabsorbed, and the absorption oil is then treated to recover a liquid` fraction therefrom including propane and heavier hydrocarbons, that combination of steps which comprises.: raising the temperature and decreasingl the pressure on said contacted absorption oil to `completely remove all absorbed hydrocarbons,

cooling said removed hydrocarbons and liquefying a portion thereof, reabsorbing said removed hydrocarbons in further absorption oil to absorb at least a portion of the propane and heavier hydrocarbons therein, passing said reabsorbed hydrocarbons in heat exchange relationship with said removed hydrocarbons, separating the reabsorbed hydrocarbons from said further absorption oil then stabilizing said separated hydrocarbons.

v'7. In a process for recovering casing-head gasoline from natural gas containing hydrocarbons of the methane series including butane and lighter hydrocarbons in which the gas is contacted with absorption oil under such conditionsV of temperature and pressure that propane and heavier hydrocarbons are absorbed but the major portion of the gas remains unabsorbed, and the absorption oil is then treated to recover a liquid fraction therefrom including propane and heavier hydrocarbons, that combination of steps which comprises: raising the temperature and decreasingthe pressure on said contacted absorption oil to completely remove all absorbed hydrocarbons, cooling said removed hydrocarbons and liquefying a portion thereof, reabsorbing from the un-4 condensed vapors the desirable hydrocarbons in further absorption oil to absorb at least a portion of the propane and heavier hydrocarbons therein, passing said reabsorbed hydrocarbons in heat exchange relationship with said removed hydrocarbons, then fractionating said further absorption oil to separate said propane and heavier hydrocarbons and stabilizing said separated hydrocarbons.

8. In a process for recovering casing-head gasoline from natural gas containing hydrocarbons of the methane series including butane and lighter hydrocarbons in which the gas is contacted with absorption oil under such conditions of temperature and pressure that propane and heavier hydrocarbons are absorbed but the major portion ofthe gas remains unabsorbed, the pressure on the absorption oil is decreased while its temperature is increased to completely remove all -absorbed hydrocarbons, said 'removed hydrocarbons arecooled and reabsorbed inv further absorption oil to separate out an undesired gaseous .fraction the major portion of which consists of propane and lighter hydrocarbons, that combination of steps which comprises: passing said gaseous fraction through absorptionoil under such conditions of temperature and pressure that substantially all propane 'and heavier hydrocarbons are absorbed therein while methane and ethane are substantially unabsorbed, thenstepping up the pressure on said absorption oil and contained gases( and contacting the same with said original gas.

JOHN LINDSAY HUTCHENBON. 

